Allow DB-specific scopes & filters by delaying until HTTP request

Resolves #2213

You can use this like so:
```ruby
ActiveAdmin.register Post do
  delay do
    Post.uniq.pluck(:category).each do |cat|
      scope(cat) { Post.where category: cat }
    end
  end
end
```

TODO:
+ add tests
+ move the code into a module to `include` into ResourceDSL
+ this is broken because it will add scopes and filters multiple times,
  once per HTTP request